Transformer control circuit



Nov. 29, 1949 F. L. BRANDT, JR 2,489,697

TRANSFORMER CONTROL CIRCUIT Filed July 10, 1946 INVENTOR.

ATTORNEY Patented Nov. 29, 1949 UNITED STATES E ATENT OFFICIE TRANSFQRMER CONTROL CIRCUIT Francis L. Brandt, 51a, Saugus, Mass, assignor to Thomson Electric Welder. (10., Lynn, Mass, a corporation of Massachusetts Application July 10, 1946, Serial No. 682,490

3 Claims. 11

This invention relates to electrical circuits, and is concerned particularly with a control circuit for developing potentials-of different values from a common source of constant potential, and for periodically shifting the-valueoi the output potential. While the control circuit ofthis invention has a variety of uses wherever a selective multiple voltage is desired, the control has been developed and is particularly useful in effecting, during a flash welding operation, a changeover in the voltage impressedacross the. electrodes of a flash Welder.

A first advantage is that use of reater voltage during the early flash period speeds up the operation by reducing thelength of the-flash period.

Furthermore, flash rwelds-have been found to show improved grain: structure and greater strength when a; voltage changeover is effected during the flash period andbefore the upset. For example, it has been: found beneficial to lower the voltage acrossthe electrodesfor. a portion of the flash period and before theupset to a value below the voltage which is most desirable for the initial portion of the flash period and/or during the period of upset. In order to attain uniform welds even as good as-those that can be obtained without voltage shift, and secure the improved welds resulting from the use of different voltages, it is imperative that the character of the weld not be adversely affected during: the voltage changeover. economically practical control circuit'for:eilecting such a changeover Without: adversely affecting the weld.

Electrodes of. resistance. flash. welders are en.- ergized from sources of constant potential, such as of 220, e40 or 550 voltsythrough transformers which convert the voltagectorthoseacceptable'ior the Welding operation. Because: of .thelarge current demand during flashing andstill larger current drawn during: upsetting; the: transformer coils are made massive; While; therefore, provision for mutiple: voltages across the secondary The present invention provides an coil of the transformer: can be." made by tapping the transformer to lessen the number of energized turns in the primaryduringtheperiod of higher voltage output, and increasing the number of energized turns in the primary during lower'voltage output, the lower number of turns must con-- age operation. Moreover; the-copperin the addi tional turns representsa. considerable investment of copper wholly idle .duringiperiods'of maximum: current demand. The presence; of these-amounts of excess or idle copperunder: both conditionsof voltage use render. sucl ral structure unreasonably expensive compared to conventional single vo1t-- age iiash welders, and henceuneconomical.

Circuits of this invention do not have any such. wasteful excess of idle or'unne'cessary copper. They utilize transformers which develop selective multiple output voltages wherein: the" total turns of the'primary coils carry theicurrent during periodsof designed: maximum voltage and. maximum current flowinthe: output circuit. and no more than these turns carry the current during periods of lesser: voltage andslesser current flow in the output circuit. Suchf transformer connection is accomplished-c by providing the primary with at least two coils. anrlconnectlngalltheturns: of each. coil in. parallel: for higher voltage secondary output and. connecting no more than the total turns, and preferably less, in series for lower voltage secondary output. The:- total amount of copperpresent in' the'series connected turnsmay'then be restricted to only that needed to carry the lower current? drawn during operation at the lower voltage in the output circuit, and the parallel connection will insure that: sufiicient copper is available to carry the higher current during operation at thehigher voltage in the output circuit. A substantial saving in copper or other conducting material results.

In order to shift;.irr such a circuit,.the voltage impressed across the electrodes during the weldin operation, I provide, in eachof' the parallel and series circuits, contactorsfor energizing the circuits separately and whereby one circuit may be open while the other. is closed. I- have'found, however, that if, between theopeningofone circuit and the closing of the other, particularly during any stepdown changeover during the flashing period, the current is interrupted by any changeover lag for a period enduring for even as small a time as one cycle, imperfectwelds result. Prevention of current interruptions of evenvthis small magnitude of time could be insured if the changeover circuit could be closedbeiorethe operating circuit is open, but in view of the connections a short would result if this operation were attempted. Instantaneous (within one cycle) opening and closing of thetwo circuits is thus necessary for good welds without circuit breakdowns. While, theoretically; instantaneous changeover can be provided by the use of thyratron controlled ignitrons or other contactors, as a practical matter, available components are not sufficiently reliable for 100% uniform instantaneous operation, and introduction into the circuits of protective time delays appears essential to prevent breakdowns.

The dilemma thus arises either of ruining the uniform perfection of the welds as a result of introducing a lag in the voltage changeover to protect the circuits, or else of endangering the circuit components in disregard of the recommendations of manufacturers of such components and against the test of experience to secure the improved welds.

The main object of the present invention, therefore, is the provision, in a transformer circuit wherein the current is carried, during operation at the designed maximum voltage and maximum current in the output, by all the copper or other coil conducting material present in the transformer, of a selective lower-voltage in the output circuit, and wherein a changeover from one voltage to the other voltage is effected with the use of commercially available components which are designed for instantaneous operation without endangering the circuit or any of its components should the components fail to operate exactly instantaneously, and without introduction of time delay devices for deliberately interrupting the current during the changeover to create a lag for protecting the circuit in the event of abnormal operation.

A further object of the invention is to provision in a flash welding control of such a circuit wherein the voltage (R. M. S. in the case of an A. C. source) across the electrodes changes from one value to another value within a period of less than one cycle, assuming a 60 cycle source.

A further object of the invention is the provision in such a transformer circuit of inductive reactance operative to limit either momentarily or for longer periods, as desired, the current below the maximum current rating (instantaneous or average, depending upon the purpose contemplated) of the components of the circuit in the event of non-instantaneous changeover from one designed voltage to another.

In one aspect, the invention has as an additional object the provision of a circuit having selective multiple voltage output of two or more voltage values wherein changeover from any one to any other voltage may be instantaneously effected and wherein during operation at the intermediate output voltage value, the current is carried by all the available copper or other conducting material in the transformer coils.

Circuits attaining the above objectives are reproduced in a conventional manner in the accompanying drawings wherein Fig. 1 is a schematic drawing of a circuit of this invention particularly adapted for two-voltage output; and Fig. 2 is a similar drawing of a circuit useful for three-voltage output. In Fig. 1, i and i2 represent two primary coils of a transformer having a common secondary coil M. Leads 15 and I6 connect the outer ends of coils I0 and i2 through a line switch l3 to a source of standard power supply H of substantially constant voltage, e. g. 220, 440, or 550 volts, preferably 60 cycle A. C. The inner ends of coils It and i2 are connected to lines It and respectively, through lines l8 and 20, thus connecting the two coils H] and I2 in parallel. Interposed in line It is an ignitron contactor 3B for making and breaking the circuit through coil [0, and a similar contactor 32 is interposed in line 20 for making and breaking the circuit through coil l2.

When contactors 3i] and 32 are both closed, coils Hi and I2 are thus connected in parallel impressing across the secondary coil I4 a voltage determined by the ratio of the number of turns in one primary coil to the number of turns in the secondary coil.

For series connection both of coils If! and I2 are tapped as at 46 and 4| and connected by a line :52 provided with an ignitron contactor 44. A series circuit is thus formed which includes a fraction (greater than one-half) of the turns of both coils i0 and I2, i. e. the groups T2, T4.

When parallel circuits l8 and 26 are open and series circuit 5'2 closed, the voltage impressed across the secondary coil M will then be determined by the ratio of the aggregate number of turns in both coils T2 and T4 energized in the series circuit, to the number of turns in the secondary coil M.

When contactors 30, 32 and 44 are all closed, a

circuit is made through lines I8, 42 and 20, and groups of turns T1, T3, which will impress across the secondary a voltage determined by the ratio of the aggregate number of turns in groups T1 and T3 of the primary coils l0 and I2 energized in this series circuit to the number of turns in the secondary coil M.

The taps 46 and 4| may be in multiple as shown so that the effective number of turns in the groups T1, T3 and T2, T4 of the series circuits may be inversely varied, thus providing a different increment between the two voltages impressed across the secondary coil thereby. Suitable means for effectively changing the taps while all the circuits are dead by opening line switch I3 may be included as indicated at 45, 46.

Secondary i4 is diagrammatically shown connected to a pair of relatively movable electrodes or Work-holders 50, 5| of a resistance welding apparatus by leads 52 and 53, respectively.

For flash welding operation, instantaneous changeover is accomplished by utilizing electronic contactors such as thyratron controlled ignitrons. In normal instantaneous operation, the secondary R. M. S. voltage thus shifts directly from the higher to the lower voltage without dropping below the lower voltage or going to zero as would be the case if time delay components interrupted the current.-

Protection against circuit breakdown in the event of abnormal non-instantaneous operation is provided by so placing taps 40 and 41 that as many turns of the coils l0 and I2 as possible are used in the series circuit made when contactors 3!] and 32 are open and contactor 44 closed, without creating, when all the contactors are closed, any circuit which will draw instantaneously current in excess of the instantaneous peak current ratings of the circuit components.

As will be understood, as the taps are moved inwardly from the inner coil ends, the greater will be the inductive reactance included in the circuit made when all three contactors are closed, and I have found that sumcient turns may be included in that circuit to provide an amount of inductance sufiicient to limit the instantaneous current drawn in the circuit at least momentarily to less than the maximum instantaneous anode current rating of commercially available electronic contactors, while still retaining sufficient coils in the series circuit made when the contactors 30 and 32 are open to provide a suitably lowsecondary voltage relative to the secondary al, itis desirable that a this lower voltage have a value between 60 and 80% of thevaludof the higher voltage. The: less. the increment between the two voltages, the greater the amount of inductive reactance included in'th'eprotective third circuit.

For illustrative purposes only. a circuit ofthis' invention connectedto a-440 volt A. 6. power sup ply might have each of coils Wand l-Z contain'a' ratio of 32 turns per 1' turn ofsecondary, thusproducing a secondaryvoltage'in parallel oper ation of about 13.8 volts (-R. M. S.) for use dur ing the initial flashperiod and possibly during the upset. Depending upon-the particular char-- actor of the work being welded, such an arrange ment might draw about 700 amperes during the initial flashing. The main series circuit might then contain in each coil turns in a ratio of about 2 of the total turns in each coil, so that the ratio of primary to secondary turns during normal series operation will be 50:1, thus impressing a voltage of about 8.8 volts (R. M. S.) across the secondary, with the work then drawing perhaps 400 amperes. This voltage is 63.8% of the higher voltage.

The protective circuit would then include 5 2 of the total turns of the coils and would impress across the secondary a voltage of about 30 volts (R. M. 8.). With such voltage, depending upon the character of the work, the amperage drawn in this circuit would be limited to perhaps 3000 amperes, which is practical for available component instantaneous ratings.

If the voltage remains at the lower value during the upset, the current might reach 150% amperes.

If during the changeover from the higher 13.8 voltage to the lower 8.8 voltage involving opening of contactors 30 and 32 and closing of contactor 44, exactly instantaneous operation of the contactors fails, with the result that all three contactors 30, 32 and 44 are momentarily closed simultaneously, no damage is done to the circuits because the impedance included in the circuit formed by lines I8, 42 and 20 limits the current to 3000 amperesbelow the maximum ratings of available circuit components. Meanwhile, the only result to the work would be a momentary application of a high voltage and current, a condition usually not as disastrous to the weld as a condition of complete current interruption during the changeover. If the number of turns included in the protective circuit is only sufficient to limit the current instantaneously drawn to less than the maximum instantaneous current ratings of the circuit components, conventional circuit breakers may be introduced into the circuit for operation should the protective circuit remain energized more than momentarily.

Fig. 2 shows a modification of the control circuit wherein, instead of three contactors as shown in Fig. 1, five contactors are utilized. The two additional contactors, 60 and 62, are interposed respectively in lines 45 and It. This type of circuit is preferred where selective third-voltage output is desired.

So long as contactors 60 and 62 are closed, the circuits operate exactly as the circuits shown in Fig. 1, but contactors 80 and B2 are included so that by opening them instantaneously whenever contactors 30, 32 and 44 are all three simultaneously closed, the secondary voltage produced by energizing coils T1 and T3 in series may be maintainediover" a: period'- of time to'provide a maintained operating: third voltage higher than the It will be understood. that in both Fig. Land Fig; 2, each of the coils lfl and it represents one or more primary coils of a single transformer or one or more coils of separate transformers, and.

that the contactors 30; 32, 44, ell-and 62 may be manually operated switches, relays or electromagnetic contactors instead of ignitron tube type contactors with or without phase shift heat control.

I claim:

1. A transformer circuit for developing selective output voltages of different values from. an electrical power source of substantially constant potential comprising a plurality of primary transformer coils, circuits including contactors for conmeeting said coils to said source in parallel, for connecting a fraction of the turns of said coils to said power source in series and means for actuating said contactors selectively to open one of said circuits and close the other of said cir cuits substantially instantaneously, the remainder of the turns of said coils being connected energized from said source in series when both said circuits are simultaneously closed to limit inductively the current instantaneously drawn from said source when both said circuits are closed below the instantaneous maximum current ratings of the components of said circuits, whereby said circuits are at least momentarily protected in the event that by reason of noninstantaneous operation of said contactors said series circuit closes before said parallel circuit opens.

2. An electrical circuit for developin selectively high and low voltages from an electrical source of substantially constant potential comprising a transformer having a plurality of primary coils and a secondary coil, a circuit including contactors for connecting said primary coils to said source in parallel to impress across said secondary coil a voltage of one value, a circuit tapping said primary coils and including a contactor for connecting a fraction of the turns of said primary coils to said source in series to lower drastically the voltage impressed across said secondary coil, and control means for actuating said contactors substantially instantaneously to shift the value of the voltage across said secondary coil from said higher to said lower value, the remainder of the turns of said primary coils being energized from said source in series when both said circuits are closed to limit inductively the current instantaneously drawn from said source when both said circuits are closed below the instantaneous maximum current ratings of the components of said circuits, whereby said circuits are at least momentarily protected in the event that by reason of non-instantaneous operation of said contactors said series circuit closes before said parallel circuit opens.

3. A transformer circuit for developin selective output voltages of different values from an electrical power source of substantially constant potential comprising a plurality of transformer 7 primary coils, a circuit for connecting said primary coils to said power source in parallel, a circuit for connecting a fraction of the turns of said coils to said power source in series, a circuit for connecting the remainder of the turns of said primary coils to said power source in series and control means for selectively energizing from said source separately said parallel circuit, said fractional turns series circuit, and said remainder turns series circuit, said last-mentioned circuit serving to limit the current instantaneously drawn from said source when both said parallel circuit and said fractional turns series circuit are simultaneously closed to a value less than the maximum instantaneous current ratings of the components of said two circuits.

FRANCIS L. BRANDT, J R.

REFERENCES CITED The following references are of record in the file of this patent:

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